.container {
  height: 100vh;
  /* 垂直方向压力 */
  display: flex;
  flex-direction: column;

  /* justify-content: space-around; */
}

.container > div {
  box-sizing: border-box;
  border: 1px solid #ff0000;
}

.container > div:nth-child(2) {
  flex: 1;
  /* 盒子嵌套布局 */
  display: flex;
  justify-content: center;
  align-items: center;
}
